bc: remove special-cased assignment to ibase, it works correctly with general rules

bc: remove special-cased assignment to ibase, it works correctly with general rules

function                                             old     new   delta
zxc_program_print                                    683     681      -2
zxc_program_prep                                      91      89      -2
zxc_program_copyToVar                                300     298      -2
zdc_program_printStream                              146     144      -2
zdc_program_execStr                                  520     518      -2
zdc_program_asciify                                  370     368      -2
zxc_program_exec                                    4016    4012      -4
zdc_program_modexp                                   694     688      -6
zxc_program_num                                     1020     995     -25
zxc_program_binOpPrep                                306     243     -63
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/10 up/down: 0/-110)          Total: -110 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>